1. Field of the Invention
This invention relates to a rapid, field-portable assay for substances, particularly drugs of abuse, that provides a positive signal in the presence of the substance.
2. Description of the Prior Art
At present, the well-publicized problems associated various drugs of abuse and the National policies associated with the "war against drugs" have led to an increased need for simple, rapid relatively inexpensive, and accurate methods for detecting drugs of abuse in fluid samples. Currently available methods and kits for testing for various chemical and biological species such as drugs of abuse, hormones, etc. suffer from the fact that these often involve relatively expensive, bulky, non-portable instrumentation. Therefore, the tests must be performed in a controlled environment.
The current systems thus often require expensive high precision instruments to measure the color change which increases the overall cost of the analysis, and requires the expertise of lab personnel. Similarly, many tests, while having a simple color change indication, involve several solutions that must be applied to a test strip. These solutions also increase the cost of the test, may require special storage conditions, increase the possibility for mistakes and also involve specialized personnel. These tests are less likely to be mobile and thus cannot be carried along or employed easily or safely by individuals conducting the test, such as lab personnel or law enforcement officers. It is thus highly desirable to develop a simple and inexpensive system which is highly mobile, yet can provide a fast, reliably accurate indication of the presence of a suspected chemical.
One enzymatic assay system using a multi-layer dry film and marketed by Eastman Kodak Co. is described in Curme et al, Clin. Chem. 24/8, 1335-1342 (1978). That system includes an upper spreading or metering layer, a reagent layer containing a binder and all required reagents (buffer, enzymes, dye precursors) beneath the metering layer and a transparent support beneath the enzyme-containing layer. The Kodak system, however, does not perform immunoassays and lacks a membranous layer for controlling the velocity of sample travel within the enzyme-containing layer. The metering layer of the Kodak system reduces the possibility of overloading of the enzyme, but does not use a membrane to effectively control the velocity at which the sample moves once it reaches the enzyme-containing layer. Thus, the sensitivity of the Kodak assays may be decreased by incomplete enzymatic reaction. Also, this lack of appropriate velocity control prevents effective use of the Kodak system for immunoassays.
Greener et al, in "Multilayer Fluorescent Immunoassay Technique", Clinical Chemistry, Vol. 35, No. 9 (1989) discloses a multilayer fluorescent immunoassay. That immunoassay uses a multilayered structure including, from top to bottom, a spreader layer, a top coat layer for filtering out potentially interfering proteins, a signal layer of antibody-indicator-hapten conjugate in an agarose matrix, and a polyester film base. The observed fluorescence is inversely proportional to the concentration of assayed substance present in the sample. Also, since fluorescence must be measured, the immunoassay requires instrumentation.